In recent years, oil wells or gas wells have been developed actively in severe environments where drilling was difficult. For example, development of a corrosive sour well which contains hydrogen sulfide and carbon dioxide in a large quantity or development of a deep well which reaches several thousands meters depth is increasingly activated.
For the drilling of such a sour well and the collection, transportation and storage of crude oil or natural gas, a steel which is excellent in corrosion resistance, particularly excellent in corrosion cracking resistance is needed. The stress corrosion cracking in an environment containing hydrogen sulfide is called sulfide stress cracking (hereinafter referred to as “SSC”).
Further, for the deepening of the wells and the improvement in transportation efficiency, a steel with high strength is needed; however, a steel with higher strength is more likely to cause SSC.
Therefore, a demand for a steel which has both more excellent strength and sulfide stress cracking resistance (hereinafter referred to as “SSC resistance”) than in the past has increased, and a steel or a steel pipe which has a higher strength and excellent SSC resistance is proposed in the Patent Documents 1 to 3, respectively.
It is disclosed in the Patent Document 1 that a technique for preventing the pitting, which starts from a coarse TiN, and consequently preventing the start of the SSC from the pitting be accomplished, by regulating the size and the precipitation amount of TiN, more specifically, by restricting the amount of TiN, which has a diameter of not less than 5 μm, to not more than 10 pieces per mm2 of the cross section, in a high strength steel pipe which has a specified chemical composition and a yield stress (hereinafter also referred to as “YS”) of not less than 758 MPa (110 ksi).
It is disclosed in the Patent Document 2 that a technique for obtaining a steel product which has a high strength of YS, between 738 and 820 MPa and excellent SSC resistance be developed, by regulating the properties of nonmetallic inclusions in a steel product which has a specified chemical composition, more specifically, by restricting the maximum length of the inclusions to not more than 80 μm and also the amount of the inclusions having a grain size of not less than 20 μm to not more than 10 pieces per 100 mm2 of the cross section.
Further, it is disclosed in the Patent Document 3 that a technique for suppressing the generation of coarse carbonitrides of Ti, Nb and/or Zr be accomplished, by forming a composite inclusion which has a specified chemical composition and also has an inner core of a Ca—Al based oxysulfide and, formed around it, an outer shell of a carbonitride of Ti, Nb and/or Zr which has a long diameter of 7 μm or less, in the amount of not less than 10 pieces per 0.1 mm2, and thereby preventing pitting from starting due to these inclusions, so as not to induce SSC starting from the pitting.
However, in the recent situation, even the techniques proposed in the Patent Documents 1 to 3 may be unable to respond to the industrial need of the development of a steel product having both high strength and increased SSC resistance.
That is to say, recently, a corrosion test in a further severe stress condition was increasingly imposed from the point of ensuring practical safety in addition to the increase in the strength of the steel products or steel pipes. The conventional target of the SSC resistance was to obtain a never fractured steel product with 758 MPa class (110 ksi class) specified minimum stress, when it was subjected to a constant load type SSC test regulated in the TM 0177-96A method of NACE (National Association of Corrosion Engineers), more specifically, when it was subjected to a constant load test with an applied stress of 80 to 85% of 758 MPa for 720 hours in an environment of 0.5% acetic acid+5% sodium chloride aqueous solution of 25° C. saturated with hydrogen sulfide of the partial pressure of 10132.5 Pa (0.1 atm).
Similarly, the conventional target of the SSC resistance was to obtain a never fractured steel product with 862 MPa class (125 ksi class) specified minimum stress, when it was subjected to a constant load test with an applied stress of 80 to 85% of 862 MPa for 720 hours in an environment of 0.5% acetic acid+5% sodium chloride aqueous solution of 25° C. saturated with hydrogen sulfide of the partial pressure of 3039.75 Pa (0.03 atm).
However, recently, it was requested that the SSC resistance, even the above-mentioned steel products, with a specified minimum stresses of 758 MPa class (110 ksi class) and 862 MPa class (125 ksi class) are never fractured when tested for 720 hours in the above-mentioned respective environments with application of the stress of 90% of YS actually possessed by each steel product (hereinafter also referred to as “actual YS”). In a condition with application of such a high stress close to the actual YS, it is difficult to suppress the SSC even if the hydrogen sulfide partial pressure is equal to or lower than the conventional condition, and it becomes more difficult to ensure the SSC resistance even with the techniques proposed in the Patent Documents 1 to 3.
In this way, the recent extremely severe test condition for the SSC resistance evaluation makes it difficult to simultaneously assign the high strength and increased SSC resistance requested for the steel products from the industry.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-131698,
Patent Document 2: Japanese Laid-Open Patent Publication No. 2001-172739,
Patent Document 3: International Patent Publication Pamphlet No. WO 03/083152.